Buying certain packs will unlock Gift Chests for your alliance. Each pack that will have a gift will show you the gift as part of the purchase. These chests are automatically "activated" and can be claimed under Alliance -> Chests -> Alliance Gift Tab within 24 hours. 

This gift chest can contain one of the following;
400-600 Gems (10%),
10,000 Hero XP (38%)
50,000 Hero XP (7%)
2x 1h Construction/Training/Research Speedup (13.5% each)
1x 3h Construction/Training/Research Speedup (5.5% each)